Catalyst system and method for preparing flame resistant materials

ABSTRACT

Presently, multifunctional carboxylic acids, such as 1,2,3,4-butanetetracarboxylic acid (BTCA) are used to bond a hydroxyl-functional organophosphorous oligomer to cotton fabric in the presence of a catalyst, such as sodium hypophosphite (NaH 2 PO 2 ). However, the free carboxylic acid groups bound to the cotton fabric form a calcium salt during home laundering, thus diminishing the flame retardant properties of the treated cotton fabric. Disclosed herein is a new catalyst system consisting of hypophosphorous acid (H 3 PO 2 ) and a nitrogen-containing organic base such as triethanol amine (TEA). When the catalyst system is present together with the polycarboxylic acid, TEA esterifies the free carboxylic acid groups under curing conditions, thus reducing calcium concentration on the fabric during home laundering. It also provides nitrogen-phosphorous synergism to enhance the flame retardant performance of the organophosphorous compound. The cotton fabric treated with BTCA and the hydroxyl-functional organophosphorous oligomer in the presence of this new catalyst system demonstrate flame retardant properties superior to that treated with NaH 2 PO 2  as a catalyst.

BACKGROUND OF THE INVENTION

[0001] The durable flame retardant finishes for cotton and other cellulosic fabrics commonly used by the industry include the tetrakis-(hydroxylmethyl)phosphonium chloride (THPC)—based system with the commercial name of “Proban”, and dimethyl (N-hydroxylmethylcarbamoylethyl) phosphonate and its analog, known as reactive organophosphorous chemicals with the trade name of “Pyrovetax CP” (1-2). However, the THPC technology requires an expensive amination chamber and strict application condition control to assure consistent performance. It is not compatible with the overwhelming majority of the existing textile finishing equipment, therefore is not considered to be practical for mass-market production. The reactive organophosphorous chemicals technology involves the use of a N-methylol phosphorous-containing flame retardant agent and a N-methylol crosslinking agent, and both compounds lead to the emission of high levels of formaldehyde, a known carcinogen, during the application of the finish to cotton fabric as well as during the use of finished cotton products by consumers. Therefore, the flame retardant chemicals for cotton commercially available to the textile industry are very limited.

[0002] Polycarboxylic acids, such as 1,2,3,4-butane-tetracarboxylic acid (BTCA), have been used as nonformaldehyde crosslinking agents for cotton and wood pulp cellulose (4-5). Alkali metal salts of phosphoric, phosphorous and hypophosphorous acids, such as sodium dihydrogen phosphate (NaH₂PO₄), sodium phosphite (Na₂HPO₃), and sodium hypophosphite (NaH₂PO₂), have been used as catalysts for the esterification and crosslinking of cellulose by polycarboxylic acids (6-9). In the presence of those catalysts, a polycarboxylic acid molecule esterifies cellulose and forms multiple ester linkages with cellulose, thus crosslinking cellulose and imparting wrinkle resistance to cotton fabrics (10).

[0003] U.S. Pat. No. 6,309,565 (Oct. 31, 2001) reports fabric treatments with a formaldehyde-free hydroxylalkyl-functional organophosphorous flame retardant compound (FR) and a cross-linking agent such as 1,2,3,4-butanetetracarboxylic acid (BTCA). BTCA apparently functions as a binding agent between the flame retardant compound and cotton cellulose. It is reported that a catalyst such as NaH₂PO₂ may be used if adequate cross-linking is to be achieved.

[0004] Because the use of cotton in apparel and home furnishing has became increasingly popular and new federal mandatory standards for fabric flammability have emerged (3), there is an urgent need to develop new and formaldehyde-free durable flame retardant finishes for cotton and cotton blends to meet the increasing demand of the market.

SUMMARY OF THE INVENTION

[0005] The present invention relates to a catalyst system for a nonformaldehyde durable flame retardant finish for fabrics. One preferred flame retardant finish comprises a hydroxylalkyl-functional organophosphorous compound (FR) and a polycarboxylic acid. The new catalyst system comprises (1) hypophosphorous acid (H₃PO₂) or salts thereof and (2) a nitrogen-containing organic base, such as triethanolamine (TEA). The nitrogen-containing organic base reacts with H₃PO₂ in an aqueous solution to form a salt of hypophosphorous acid (Scheme 1), which functions as the catalyst for the esterification of a carboxylic acid with cellulose and FR.

[0006] More specifically, provided is a method of binding a flame retardant compound or composition to cellulose comprising: applying a composition comprising a hydroxyl-functional flame retardant, a polycarboxylic acid, hypophosphorous acid and a nitrogen-containing organic base to a cellulose-containing material. This method may further comprise curing the cellulose-containing material. Also provided is a catalyst system for bonding flame retardants to fabric through a polycarboxylic acid comprising hypophosphorous acid and a nitrogen-containing organic base.

[0007] As used herein, “polycarboxylic acid” includes any organic structure with more than one carboxylic acid functional group. Some examples of polycarboxylic acids include 1,2,3,4-butanetetracarboxylic acid, citric acid, poly(maleic acid), poly(itaconic acid), copolymer of maleic acid and itaconic acid, poly(fumaric acid) or mixtures of two or more of these acids. As used herein, “nitrogen containing organic base” does not include ammonia and other bases that do not contain carbon. A preferred nitrogen containing organic base is triethanolamine (TEA).

[0008] Various fabrics and materials can be treated with the compositions and methods of the invention as long as they contain cellulose. Various salts of hypophosphorous acid may be used, as known in the art. The flame retardant compound is any of a number of flame retardants known in the art, such as a hydroxylalkyl-functionalized organophosphorous compounds.

[0009] Monomeric, oligomeric (which generally contain from about two to ten repeat units) and polymeric (which generally contain over about ten repeat units) hydroxyalkyl-functional organophosphorus flame retardant additives are intended for use herein.

[0010] A reactive oligomeric phosphorus-containing flame retardant of the type that is described in U.S. Pat. No. 3,695,925 to E. D. Weil and U.S. Pat. Nos. 4,199,534, 4,268,633, and 4,335,178 to R. B. Fearing is an example of one of the hydroxyalkyl-functional organophosphorus flame retardants that can be used in accordance with the present invention. A preferred embodiment has the following structure:

[0011] where R₁ is independently selected from methyl and hydroxyethyl, R₂ is independently selected from methyl, methoxy, and hydroxyethoxy, and n is equal to or greater than 1. This embodiment is made by a multistep process from dimethyl methylphosphonate, phosphorus pentoxide, ethylene glycol, and ethylene oxide and is available under the registered trademark FYROL® 51 from Akzo Nobel Chemicals Inc. The endgroups are principally hydroxyl groups.

[0012] Another class of materials for use herein includes water soluble oligomeric alkenylphosphonate materials, examples of which are described in U.S. Pat. Nos. 3,855,359 and 4,017,257, both to E. D. Weil. The presence of alkenyl substituents in these materials provide an additional mechanism for permanence utilizing free radical curing conditions (described in the patents above). A preferred species of this type is available under the trademark PYROL® 76 from Akzo Nobel Chemicals Inc. and is produced by reacting bis(2-chloroethyl) vinylphosphonate and dimethyl methylphosphonate with the substantial elimination of methyl chloride.

[0013] Another type of hydroxyalkyl-functional organophosphorus flame retardant that can be employed are oligomeric phosphoric acid esters that carry hydroxyalkoxy groups as described in U.S. Pat. Nos. 2,909,559, 3,099,676, 3,228,998, 3,309,427, 3,472,919, 3,767,732, 3,850,859, 4,244,893, 4,382,042, 4,458,035, 4,697,030, 4,820,854, 4,886,895, 5,117,033, and 5,608,100.

[0014] Although Applicant does not wish to be bound by theory, it is believed the nitrogen-containing organic base is bound to cotton through its esterification with the polycarboxylic acid. It also has the following functions:

[0015] (1) It provides phosphorous-nitrogen synergism for the FR compound, thus improving the performance of FR.

[0016] (2) It reacts with the free carboxylic acid groups of the polycarboxylic acid on cotton fabric under curing conditions, and significantly increases the amount of ester and reduces the formation of calcium salts of the carboxylic acid on the cotton fabric. The introduction of positive charge to the cotton fabric through TEA is thought to also replace calcium cations and prevent them from forming salt with the free carboxylic acid groups on the fabric. It was found that the formation of calcium salt on the cotton fabric treated with FR and BTCA during home laundering diminishes the performance of FR on the fabric. The reduced calcium concentration on the fabric as a result of the use of the nitrogen-containing organic base enhances the flame retardant performance of the treated cotton fabric during home laundering.

[0017] (3) It raises the pH of a finish solution, therefore improves the strength retention of the treated cotton fabric.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1. The calcium concentration on the cotton fabric treated with 9.6% BTCA and 4.8% NaH₂PO₂, cured at 185° C. for 2 min, and finally treated with CaCl₂ solutions of different concentrations.

[0019]FIG. 2. The calcium concentration on the cotton fabric treated with 9.6% BTCA and 4.8% NaH₂PO₂, cured at 185° C. for 2 min, and finally washed in tap water as function of the HLWD cycles.

[0020]FIG. 3. The pH of the cotton fabric treated with 9.6% BTCA/4.8% NaH₂PO₂ suspended in water as a function of the added volume of the 0.10 M CaCl₂ solution.

[0021]FIG. 4. The calcium concentration on the cotton fabric treated with 24% FR, 9.6% BTCA and 4.8% NaH₂PO₂, cured at 185° C. for 2 min, and finally treated with CaCl₂ solutions as a function of the calcium concentration of the CaCl₂ solutions.

[0022]FIG. 5. The calcium concentration on the cotton fabric treated with 24% FR, 9.6% BTCA and 4.8% NaH₂PO₂ and cured at 185° C. for 2 min as a function of the number of the HLWD cycles.

[0023]FIG. 6. The titration curve of H₃PO₂.

[0024]FIG. 7. The ester carbonyl band intensity of the cotton fabric treated with 24% FR, 9.6% BTCA, % H₃PO₂ in combination with different concentrations of TEA, and cured at 185° C. for 2 min as a function of the TEA concentration.

[0025]FIG. 8. The calcium concentration on the cotton fabric treated with 24% FR, 9.6% BTCA, 7% H₃PO₂ in combination with TEA of different concentrations, and cured at 185° C. for 2 min, and finally treated with 0.5M CaCl₂ for 30 min as a function of the TEA concentration.

[0026]FIG. 9. The LOI (%) of the cotton fabric treated with 24% FR, 9.6% BTCA, 7% H₃PO₂ in combination with TEA of different concentrations, and cured at 185° C. for 2 min and finally subjected to 5 HLWD cycles as a function of the TEA concentration.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The following nonlimiting examples will assist in understanding the invention.

[0028] Materials

[0029] The fabrics used in the investigation include: (1) a dark brown 100% cotton twill weave fabric weighing 246 g/m²; (2) a white 60/40 cotton/polyester blend plain weave fabric weighing 136 g/m². The flame retardant agent (FR) was a hydroxyl-functional organophosphorous oligomer with the trade name of Fyrol 51 supplied by Akzo Nobel Chemical Corporation, New York. BTCA, hypophosphorous acid (H₃PO₂), triethanolamine (TEA), and sodium hypophosphite (NaH₂PO₂) were reagent-grade chemicals supplied by Aldrich, Wis. The melamine-formaldehyde crosslinker with the trade name of Ecco Rez M-300 was supplied by Eastern Color & Chemical Company, Greenville, S.C.

[0030] Fabric Treatment and Home Laundering Washing/Drying (HLWD) Procedures

[0031] The fabric was first immersed in a finish solution containing FR, BTCA, and the catalyst, then passed through a laboratory padder with two dips and two nips, dried at 90° C. for 3 min, and finally cured in a Mathis curing oven at a specified temperature. All the concentrations presented here are based on weight (w/w, %). The wet pick-up of the cotton and cotton/polyester blend fabrics was approximately 85 and 80%, respectively. After curing, the treated fabric was first subjected to a washing/drying cycle without use of a detergent (specified here as “water wash”) to remove FR and BTCA not bound to cotton and the catalyst. The home laundering wash/dry process was done according to AATCC Test Method 124-1996 (Appearance of Fabrics After Repeated Home Laundering). The detergent used was a commercial Tide detergent without bleach. The water temperature was approximately 45° C.

[0032] Fabric Performance Evaluation

[0033] The vertical flammability of treated cotton fabric was measured according to ASTM Standard Method D6413-99. The limited oxygen index (LOI) of the treated cotton fabric was measured according to ASTM Standard Method D2863-97. The breaking strength in filling direction of the treated cotton fabric was measured according to ASTM Standard Method D5035-95.

[0034] Infrared Spectroscopy Measurement

[0035] All the infrared spectra presented are diffuse reflectance spectra collected with a Nicolet Magna spectrometer and a Specac diffuse reflectance accessory, and are presented in absorbance mode (−log R/R₀) for quantitative analysis. Resolution for all the infrared spectra is 4 cm⁻1, and there were 100 scans for each spectrum. Potassium bromide powder was used as a reference material to produce a background diffuse reflectance spectrum. The treated and cured cotton fabric was first washed in water to remove FR and BTCA not bound to cotton and the catalyst, then treated with a 0.1 M NaOH solution at room temperature for 4 min to convert the free carboxylic acid group on the fabric to a carboxylate anion. The fabric sample thus treated is dried at 80° C. for 5 min. To improve sample uniformity, the fabric sample was finely ground in a Wiley mill to form a powder before infrared spectroscopy analysis. The ester carbonyl band intensity in the infrared spectra was normalized against the 1318 cm⁻1 band associated with a C—H bending mode of cellulose.

[0036] Determination of Phosphorous and Calcium Concentration on the Treated Cotton Fabric

[0037] Approximately 2 g of treated cotton fabric taken from different parts of a larger fabric specimen were ground in a Wiley mill into a powder to improve sample uniformity. 2 ml concentrated H₂SO₄ were added to 0.1 g of cotton powder. 10 ml 30% H₂O₂ were added dropwise to the mixture, allowing the reaction to subside between drops. The reaction mixture was then heated on a hotplate at approximately 250° C. to digest the powder and to evaporate the water until dense SO₃ vapor is produced. The completely digested cotton sample as a clear solution was transferred to a 50-ml volumetric flask, then diluted with distilled/deionized water. The sample thus prepared was analyzed with a Thermo-Farrell-Ash Model 965 induced current plasma atomic emission spectrometer (ICP/AES) to determine the % concentrations of phosphorous and calcium.

[0038] Formation of Calcium Salt on the Cotton Fabric Treated with BTCA

[0039] The cotton fabric treated with 9.6% BTCA and 4.8% NaH₂PO₂ was cured at 185° C. for 2 mm. The treated fabric was first washed in water to remove the catalyst and BTCA not bound to cotton, then treated in CaCl₂ solutions of different concentrations at room temperature for 30 min. The calcium concentration of the CaCl₂ solutions ranged from 0.10 to 4.00%. The cotton fabric thus treated was thoroughly washed in deionized water for 30 min to remove any residual calcium ions not bound to the fabric, and finally dried.

[0040] The calcium concentration on the cotton fabric determined by ICP/AES is plotted against the calcium concentration of the CaCl₂ solutions used to treat the fabric (FIG. 1). One observes that the calcium concentration on the fabric increased as the calcium concentration of the solution increased (FIG. 1). The calcium concentration on the fabric reached approximately 0.3% when the calcium concentration of the solution was increased to 1.00%, and it stabilized at the 0.3% level as the calcium concentration of the solution increased further (FIG. 1). Thus, the data indicate that the calcium cations form salt with the free carboxylic acid group on the fabric, which has low solubility in water (Scheme 3). The formation of calcium salt on the treated cotton fabric reached saturation when the calcium concentration in the solution was increased to 1.00% as shown in FIG. 1.

[0041] The cotton fabric treated with 9.6% BTCA/4.8% NaH₂PO₂ and cured at 185° C. for 2 min was also washed in tap water in the presence of a detergent. The calcium concentration on the cotton fabric is plotted against the number of the home laundering washing/drying (HLWD) cycles (FIG. 2). One observes that the calcium concentration on the fabric increased as the number of HLWD cycle was increased, and it reached approximately 0.28% after 5 HLWD cycles (FIG. 2). The data show that the calcium cations of the tap water form salt with the free carboxylic acid group bound to the treated cotton fabric.

[0042] The cotton fabric treated with 9.6% BTCA/4.8% NaH₂PO₂ and cured at 185° C. for 2 min was ground into a powder. 0.1 g of the powder sample was suspended in 50 ml distilled water, and then titrated with a 0.10 M CaCl₂ solution. The pH of the fiber/water mixture was plotted against the volume of the CaCl₂ solution added to the mixture (FIG. 3). The pH of the fiber/water mixture decreased as the volume of the added CaCl₂ solution increased. The steady decline in pH value was evidently a result of the formation of calcium salt on the fiber as shown in Scheme 3, which liberated the proton from the carboxylic acid groups on the fabric. The pH value stabilized at approximately 4.65 when the volume of the CaCl₂ solution was increased to 14.0 ml, indicating the formation of calcium salt on the fiber reached saturation. All the data demonstrated that calcium cation reacts with the free carboxylic acid bound to the cotton fabric to form insoluble salt.

[0043] Formation of Calcium Salt on the Cotton Fabric Treated with FR and BTCA

[0044] The cotton fabric was treated with 24% FR, 9.6% BTCA, and 4.8% NaH₂PO₂, and then cured at 185° C. for 2 min. The treated fabric was washed in water to remove the FR and BTCA not bound to cotton. The fabric was then treated in CaCl₂ solutions of different concentrations at room temperature for 30 min. The calcium concentration on the cotton fabric is plotted against the calcium concentration of the CaCl₂ solutions (FIG. 4). One observes that the calcium concentration on the treated cotton fabric increased as the calcium concentration of the solution increased, and it stabilized at 0.4-0.5% when the calcium concentration in solution was increased to 0.50% and above. The cotton fabric treated with FR and BTCA was also washed in tap water in the presence of a detergent. The calcium concentration on the cotton fabric increased as the number of the HLVVD cycles was increased as shown in FIG. 5. All the data presented above show that the free carboxylic acid group on the fabric treated with FR and BTCA also form insoluble calcium salt (Scheme 4).

[0045] The cotton fabric was treated with 16% FR and BTCA of different concentrations. The fabric thus treated was subject to 10 HLWD cycles, followed by thoroughly rinsing in deionized water for 30 min to remove the calcium physically absorbed on the fabric. The calcium concentration on the treated cotton fabric before and after 10 HLWD cycles are presented in Table 1. The calcium concentration on the fabric before washing is insignificant, and it became substantially larger after 10 HLWD cycles. One also observes that the calcium concentration after laundering also increased as the BTCA-to FR ratio was increased. Evidently, larger number of carboxylic acid groups on the cotton fabric as a result of higher BTCA concentration used to treat the fabric led to the increased calcium concentration after home laundering.

[0046] The cotton fabric was treated with 24% FR, 9.6% BTCA and 4.8% NaH₂PO₂, cured at 185° C. for 2 min, then subjected to different number of HLWD cycles. The LOI, char length, percent phosphorous retention, and calcium concentration of the fabric thus treated is shown in Table 2. In spite of the fact that the fabric still retained 89% of phosphorous on the fabric after 3 HLWD cycles, the char length exceeded 300 mm and LOI also decreased significantly. One observes that the calcium concentration increased from 0.008% to 0.110% after 3 HLWD cycles (FIG. 5). The diminished flame resistance of the treated cotton fabric is due to the formation of calcium salt of carboxylic acid on the fabric during the laundering process. The combustion and pyrolysis of cotton ultimately converts FR to phosphoric acid, which leads to dehydration of cellulose via phosphorylation-dephosphorylation cycle and retards burning (12). The calcium ions on the fabric react with phosphoric acid and form calcium phosphate, which does not function as a flame retardant agent. Consequently, the flame-resistance of the treated cotton fabric deteriorates as the amount of calcium bound to the cotton fabric increases.

[0047] When the cotton fabric was treated with 24% FR and 11.2% melamine-formaldehyde crosslinker (M-F), and cured at 165° C. for 2 min. The treated cotton fabric was then subjected to different number of HLWD cycles. The LOI, char length, percent phosphorous retention and calcium concentration of the fabric thus treated is presented in Table 3. The data indicated that the calcium concentration of the fabric remained practically unchanged during the home laundering process. After 5 HLWD cycles, the LOI for the cotton fabric treated using M-F as a crosslinking agent was 29.2 with 65% of original phosphorous retained on the fabric, whereas LOI of the fabric crosslinked by BTCA was only 24.7 with 84% of original phosphorous on the fabric. The BTCA treated fabric failed the vertical flammability test after 3 HLWD cycles, whereas the char length for the M-F treated fabric was only 135 mm after 15 HLWD cycles in spite of the fact that only 62% of phosphorous remained on the fabric. Evidently, the formation of insoluble calcium salt on the fabric is associated with the free carboxylic acid groups, not with any phosphate group formed as a result of possible hydrolysis of FR on the fabric. It was concluded that when a polycarboxylic acid is used a crosslinking agent for the FR, the free carboxylic acid group form insoluble calcium salt during the laundering process, thus diminishing the flame retardant properties of the treated cotton fabric.

[0048] New Catalyst System

[0049] NaH₂PO₂ has been the most effective catalyst for esterification and crosslinking of cotton by a polycarboxylic acid. In this research, the combination of H₃PO₂ and TEA was used as a new catalyst system to replace NaH₂PO₂. 20 ml of a 0.30 M H₃PO₂ was titrated with 0.30 M TEA. The pH of H₃PO₂ is presented as a function of the volume of TEA added (FIG. 6). H₃PO₂ is a relatively strong acid with K_(a) value of 5.9×10⁻² whereas TEA is a weak base with K_(b) value of 5.75×10⁻7. The original pH of H₃PO₂ was 1.56 before the titration was started. H₃PO₂ was neutralized to form TEA salt as TEA was gradually added as shown in Scheme 1.

[0050] The titration reached equivalent point when the volume of added TEA reached approximately 20.30 ml. The pH was drastically increased from 2.98 to 6.01 around the equivalent point (pH=4.8) when only 1.00 ml of TEA (from 19.80 to 20.80 ml) was added. (FIG. 6). Based on the pH titration curve, it was calculated that approximately 97% of H₃PO₂ were neutralized when the pH reached 3.0. Since all the finish solutions containing H₃PO₂/TEA were maintained at pH 3.0, the overwhelming majority of the catalyst is in the form as TEA salt of H₃PO₂.

[0051] The esterification of cotton cellulose by a polycarboxylic acid proceeds in two steps: formation of a 5-membered cyclic anhydride intermediate by dehydration of two adjacent carboxylic acid groups, and the reaction between cellulose and the anhydride intermediate to form ester (12-13). In previous research, it was found that TEA esterifies the anhydride intermediate formed on the cotton fabric treated with a polycarboxylic acid under curing conditions. Consequently, the amount of anhydride not reacted decreased whereas the amount of ester on the fabric increases (14).

[0052] The cotton fabric was treated with 24% FR, 9.6% BTCA, 7% H₃PO₂ in combination with different concentrations of TEA. The pH of all the finish solutions was adjusted to 3.0 using either NaOH or HCl solutions. The fabric was cured at 185° C. for 2 min, and washed in deionized water to remove any FR, BTCA and TEA not bound to cotton. Before the fabric samples were analyzed by FT-IR spectroscopy, the samples were treated with 0.1 M NaOH to convert the free carboxylic acid groups to carboxylate anions so that the ester carbonyl band was not overlapped by the carboxylic carbonyl, therefore could be measured quantitatively (15). The ester carbonyl of the cotton fabric thus treated is plotted against the TEA concentration in FIG. 7. The amount of ester formed on the treated cotton fabric increased notably as the TEA concentration was increased, and the ester carbonyl band intensity reached its maximum when the TEA concentration was increased to 8%. The infrared spectroscopy data evidently show that addition of TEA to the finish system resulted in esterification of BTCA with TEA during the curing process, thus increasing the total amount of ester formed on cotton. A further increase in the TEA concentration from 8% to 14% reduces the ester carbonyl band intensity (FIG. 7). This was because that both TEA and cotton cellulose competed to react with BTCA, a further increase in the esterification between TEA and BTCA led to the decrease in the esterification between cotton and BTCA, thus reducing the total ester bound to cotton.

[0053] The calcium concentration of the cotton fabric treated with 24% FR, 9.6% BTCA, 7% H₃PO₂ in combination with different concentrations of TEA and cured at 185° C. for 2 min was treated in a 0.5 M CaCl₂ solution for 30 mins. The calcium concentration of the fabric thus treated is plotted against the TEA concentration (FIG. 8). One observes a significant decrease in calcium concentration as the TEA concentration was increased, and the calcium concentration reached its minimum when the TEA concentration was increased to 8%. Previously, it was found that ester formation reached its maximum when TEA concentration was increased to 8%. The reduction in calcium concentration on the fabric as shown in FIG. 8 is obviously due to the reduction in the amount of free carboxylic acid on the treated cotton fabric as a result of increased esterification of BTCA by TEA. One also observes that further increase in the TEA concentration from 8% to 14% in the finish bath did not cause further reduction in calcium concentration.

[0054] The LOI (%) for the cotton fabric treated with FR, BTCA and H₃PO₂ without the presence of TEA was only 24.1. It increased to 30.7 when 5% TEA was presented in the finish and it reached its maximum (31.1) when TEA concentration was 10%. The same trend remained after the treated cotton fabric was subject to different number of home laundering cycles. The LOI (%) of the treated cotton fabric after 5 laundering cycles is presented as a function of the TEA concentration in the finish system (FIG. 9). The increased LOI is attributed to two factors: the reduction in calcium concentration as shown in FIG. 8 and the increase in nitrogen concentration as a result of more TEA bonding to cotton through its esterification with BTCA. The data also indicate that a further increase to 12% in TEA concentration reduced the LOI (FIG. 9). This is consistent with the change in ester carbonyl band intensity as demonstrated in FIG. 7.

[0055] Based on the data presented above, it was concluded that the TEA added to the finish system esterifies the free carboxylic acid group, thus reducing the calcium concentration on the treated fabric. The TEA bound to cotton through its esterification with cellulose also provides phosphorous-nitrogen synergism. Consequently, the H₃PO₂/TEA catalyst system significantly enhances the flame retardant properties of the treated cotton fabric.

[0056] The Performance of the Cotton Fabric Treated with FR, BTCA, and H₃PO₂/TEA

[0057] The cotton fabric was treated with 28% FR, 14% BTCA and 7% H₃PO₂ in combination with TEA of different concentrations. The pH of the finish solutions was adjusted to 3.0 using either NaOH or HCl solutions. The treated fabric was cured at 185° C. for 2 min. The calcium concentration and LOI of the fabric treated with different TEA concentrations are presented in Table 4. The data show that calcium concentration increased as the number of laundering cycles were increased. When TEA is present, however, the calcium concentration on the fabric was drastically reduced and LOI was increased. One also observes that TEA concentration at 8-10% resulted in the highest LOI. This is consistent with the data presented in FIG. 9.

[0058] The same FR/BTCA/H₃PO₂ finishing system with TEA concentration ranging from 5% to 12% was applied to a 60/40 cotton/polyester blend fabric. The treated fabric was cured at 185° C. for 2 min. The calcium concentration and LOI of the cotton fabric thus treated is presented in Table 5. It is interesting to note that the finishing system considerably improved the flame retardant performance of the polyester/cotton blend. The data also show that TEA concentration of 10% led to highest LOI and the lowest calcium concentration.

[0059] The performance of the FR/BTCA finish with different catalysts was compared. The cotton fabric was treated with 28% FR and 14% BTCA in the presence of two different catalysts: (1) 7% H₃PO₂ in combination of 10% TEA and (2) 7% NaH₂PO₂. The fabric was then cured at 185° C. for 2 min. The calcium concentration, LOI, and char length of the cotton fabric thus treated are presented in Tables 6, 7, and 8, respectively. The calcium concentration on the fabric after 5 HLWD cycles was increased to 0.196% when NaH₂PO₂ was used as the catalyst, whereas it was only 0.045% when H₃PO₂/TEA was used. Evidently, the esterification of TEA with BTCA reduces the formation of calcium salt on the fabrics. The fabric treated with FR, BTCA, and H₃PO₂/TEA showed considerably higher LOI and shorter char length than that treated with FR, BTCA, and NaH₂PO₂ (Tables 7 and 8, respectively). The treated fabric failed the vertical flammability test after 3 HLDW cycles when NaH₂PO₂ was used as the catalyst, whereas the char length was only 91 mm after 5 HLWD cycles when H₃PO₂/TEA was used (Table 8). The data clearly demonstrate that the combined effects of reduced calcium concentration on the fabric and phosphorous-nitrogen synergism significantly improved the flame retardant properties of the treated cotton fabric.

[0060] Although the description contained herewith contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently-preferred embodiments of this invention. For example, compounds other than those specifically mentioned may be used and are included in the invention, as long as they perform the same function. Also, times and concentrations other than those specifically described may be used and are included in the invention. All references cited herein are hereby incorporated by reference to the extent not inconsistent with the disclosure herewith.

REFERENCES

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[0070] 10. C. Q. Yang and D. Wang, “Evaluation of Ester Crosslinking of Cotton Cellulose by A Polycarboxylic Acid Using Acid-Base Titration”, Textile Res. J., 70, 615-620 (2000).

[0071] 11. J. K. Stowell, E. W. Weil, W. L. Coble, “Formaldehyde-Free Flame Retardant Treatment for Cellulose-Containing Materials”, U.S. Pat. No. 6,309,545 (Oct. 31, 2001).

[0072] 12. M. Levin, “Flame Retardance of Fabrics”, in “Handbook of Fiber Science and Technology: Chemistry Processing of Fibers and Fabrics”, Vol. 2, Part B., ed., M. Lewin and S. B. Sello, Marcel Dekker, New York, 1984, p86.

[0073] 13. C. Q. Yang, “Ft-IP Spectroscopy Study of the Ester Crosslinking Mechanism of Cotton Cellulose” Textile Res. J. 61, 433-440, 1991.

[0074] 14. C. Q. Yang, “Infrared Spectroscopy Studies of the Cyclic Anhydride as the Intermediate for Ester Crosslinking of Cotton Cellulose by Polycarboxylic Acids. I. Identification of the Cyclic Anhydride Intermediate”, J. Polym. Sci., Part A. Polym. Chem. 33, 1187-1193 (1993).

[0075] 15. C. Q. Yang, “Infrared Spectroscopy Studies of the Cyclic Anhydride as the Intermediate for Ester Crosslinking of Cotton Cellulose by Polycarboxylic Acids. III. Molecular Weight of A Crosslinling Agent”, J. Polym. Sci., Part A. Polym. Chem. 35, 557-564 (1997).

[0076] 16. C. Q. Yang and G. Bakshi, “Quantitative Analysis of the Nonformaldehyde Durable Press Finish on Cotton Fabric: Acid-Base Titration and Infrared Spectroscopy” Textile Res. J. 66, 377-384 (1996). TABLE 1 The Calcium Concentration on the Cotton Fabric Treated FR and BTCA with different BTCA-to-FR Ratio FR BTCA Ca Concentration (%) Concentration Concentration BTCA-to-FR before after 10 (%) (%) Ratio washing HLWD cycles 16 2.0 0.125 0.008 0.191 16 4.0 0.250 0.000 0.202 16 6.0 0.375 0.000 0.262 16 8.0 0.500 0.005 0.302 16 12.0  0.750 0.006 0.353

[0077] TABLE 2 The LOI, Char Length and Percent Phosphorus Retention of the Cotton Fabric Treated with FR and BTCA. Number of HLWD Cycles before water after water Fabric Property wash wash 1 3 5 10 15 LOI (%) 29.0 26.5 25.8 25.3 24.7 24.1 23.2 Char Length (mm) 85 144 176 >300 >300 >300 >300 % Phosphorus Retention — 96 88 89 84 82 75 Calcium Concentration(%) — — 0.110 0.102 0.196 0.240 0.297

[0078] TABLE 3 The LOI, Char Length, Percent Phosphorus Retention and Calcium Concentration of the Cotton Fabric Treated with FR and Hydroxymethylated Melamine. Number of HLWD Cycles after Fabric Property water wash 1 5 10 15 LOI (%) 32.1 29.7 29.2 28.7 27.8 Char Length (mm) 79 84 81 94 135 % Phosphorus — 70 65 66 62 Retention Calcium — 0.044 0.039 0.029 0.024 Concentration (%)

[0079] TABLE 4 The LOI and calcium concentration of the 100% Cotton Fabric Treated with 28% FR, 14% BTCA and 7% H₃PO₂ in the presence of Different Concentration with TEA LOI(%) Calcium concentration(%) TEA After After 1 After 5 After After 1 After 5 Concentration(%) water wash HLWD cycle HLWD cycles water wash HLWD cycle HLWD cycles 0 24.1 23.6 23.5 0.041 0.101 0.196 5 30.7 29.2 27.9 0.015 0.019 0.056 8 30.9 30.2 28.1 0.003 0.011 0.021 10  31.1 30.0 28.6 0.003 0.011 0.045 12  30.2 29.7 26.3 0.006 0.007 0.031

[0080] TABLE 5 The Calcium Concentration and LOI of the 40/60 Polyester/cotton Fabric Treated with 28% FR, 14% BTCA and 7% H₃PO₂ in the presence of Different Concentration with TEA LOI(%) Calcium concentration(%) TEA After After 1 After 5 After After 1 After 5 Concentration(%) water wash HLWD cycle HLWD cycles water wash HLWD cycle HLWD cycles  5 27.4 26.3 25.2 0.032 0.034 0.064  8 27.6 26.7 25.7 0.001 0.012 0.037 10 28.6 27.7 25.8 0.007 0.006 0.010 12 27.6 26.3 24.7 0.000 0.008 0.035

[0081] TABLE 6 The Calcium Concentration on the Fabric Treated with FR and BTCA in the presence of Different Catalyst System Calcium Concentration (%) FR BTCA Catalyst Before After After 1 After 5 Concentration Concentration Concentration pH Water wash Water wash HLWD cycle HLWD cycles 28% 14% H₃PO₂ 7% 3.0 0.000 0.003 0.011 0.045 adjusted by TEA 28% 14% NaH₂PO₂ 7% 2.8 0.002 0.040 0.110 0.196 adjusted by NaOH

[0082] TABLE 7 The LOI of the Fabric Treated with FR and BTCA in the presence of Different Catalyst System LOI(%) FR BTCA Catalyst Before After After 1 After 5 Concentration Concentration Concentration pH Water wash Water wash HLWD cycle HLWD cycles 28% 14% H₃PO₂ 7% 3.0 34.3 31.1 30.0 28.6 adjusted by TEA 28% 14% NaH₂PO₂ 7% 2.8 29.2 26.9 25.8 24.7 adjusted by NaOH

[0083] TABLE 8 The Char Length of the Fabric Treated with FR and BTCA in the presence of Different Catalyst System Char Length(mm) FR BTCA Catalyst Before After After 1 After 5 Concentration Concentration Concentration pH Water wash Water wash HLWD cycle HLWD cycles 28% 14% H₃PO₂ 7% 3.0 66  70  78  91 adjusted by TEA 28% 14% NaH₂PO₂ 7% 2.8 88 133 176 >300 adjusted by NaOH 

I claim:
 1. A method of binding a flame retardant compound or composition to cellulose comprising: applying a composition comprising a hydroxyalkyl-functionalized flame retardant, a polycarboxylic acid, hypophosphorous acid and a nitrogen-containing organic base to a cellulose-containing material.
 2. The method of claim 1, further comprising curing the cellulose-containing material at a temperature of between 200 to 100° C. for a time of between 20 seconds and 10 minutes.
 3. The method of claim 1, wherein said flame retardant is a hydroxylalkyl-functionalized organophosphorous compound.
 4. The method of claim 3, wherein said hydroxylalkyl-functionalized organophosphorous compound is Fyrol
 51. 5. The method of claim 1, wherein said polycarboxylic acid is 1,2,3,4-butanetetracarboxylic acid.
 6. The method of claim 1, wherein said nitrogen-containing organic base is TEA.
 7. The method of claim 1, wherein said nitrogen-containing organic base is hydroxylalkyl functionalized.
 8. The method of claim 1, wherein said flame-retardant is present in the composition at 2 to 40 weight percent.
 9. The method of claim 1, wherein said polycarboxylic acid is present in the composition at 1 to 20 weight percent.
 10. The method of claim 1, wherein said hypophosphorous acid is present in the composition at 1 to 12 weight percent.
 11. The method of claim 1, wherein said nitrogen-containing organic base is present in the composition at a weight percent which maximizes the esterification of cellulose-containing material.
 12. The method of claim 11, wherein said nitrogen-containing organic base is present in the composition at 1 to 15 weight percent.
 13. The method of claim 1, further comprising adjusting the pH to between 2 to 4.5.
 14. A catalyst system for applying flame retardants to fabric comprising hypophosphorous acid and a nitrogen containing organic base.
 15. The catalyst system of claim 14, wherein said nitrogen containing organic base is triethanolamine.
 16. The catalyst system of claim 14, wherein said nitrogen containing organic base is selected from the group consisting of: triethanolamine, diethanolamine, and ethanolamine. 